Taxation of income received by way of enhancement

An assessee files his return showing income from rent on receipt basis. He received any amount, say, 1,00,000 in the current F.Y. 2012-13 by way of enhancement for say last 6 yrs as the rates of rent were enhanced with retrospective effect.

1. Will the sum of rupees 1,00,000 be taxable in the A.Y. 2013-14?

2. Can it be spread over the last six years?

3. Is there any provision of tax relief like in sec 89(1)??

4. What are the provisions of the Act governing such cases?